An antiferroelectric liquid crystal is stable in an antiferroelectric phase or state when left in a condition that no voltage is applied to the liquid crystal (namely, the voltage to be applied to the crystal (substance) is zero). Hereinafter, this stable state will be referred to as a neutral state. An antiferroelectric liquid crystal panel may be configured in such a manner as to effect either a dark display or a bright display in this neutral state. Although antiferroelectric liquid crystal panels of the present invention be applied to both a dark display and a bright display, an antiferroelectric liquid crystal panel of the present invention which is adapted to effect a dark display in the neutral state will be described hereinbelow.
FIG. 16 is an example of a graph illustrating the optical transmittance of an antiferroelectric liquid crystal relative to a voltage applied thereto. In this graph, the abscissa represent the applied voltage; and the ordinate the optical transmittance.
When applying a positive voltage to the crystal, which has been in the neutral state at a point 0, and increasing the positive voltage, the transmittance abruptly increases at a voltage Ft. Then, the transmittance reaches nearly the maximum value at a voltage Fs. Consequently, the crystal is put into a saturated ferroelectric state. Thence, the optical transmittance does not change much even when a higher voltage is applied thereto. Next, when the applied voltage is gradually decreased, the optical transmittance abruptly drops at a voltage At. Further, the transmittance nearly reaches zero at the voltage As. Thus, the crystal returns to an antiferroelectric state. Similarly, if a negative voltage is applied to the crystal when the applied voltage is 0 V, and the applied negative voltage is made more negative, the transmittance abruptly rises at a voltage (-Ft). Then, the transmittance nearly reaches the maximum value at a voltage (-Fs). Thus the crystal is put into a saturated ferrorelectric state. Thence, when gradually the applied negative voltage is reduced to 0 V, the transmittance abruptly drops at a voltage (-At). Further, the transmittance becomes almost zero at a voltage (-As). Thus, the crystal returns to the antiferroelectric state. As above described, there are two causes for a ferroelectric state of the liquid crystal. Namely, one is the application of the positive voltage, and the other is the application of the negative voltage. Hereunder, the ferroelectric state due to the former cause will be referred to as (+) ferroelectric state, while the ferroelectric state due to the latter cause will be referred to as (-) ferroelectric state. Further, .vertline.Ft.vertline. designates a ferroelectric threshold voltage; .vertline.Fs.vertline. a ferroelectric saturation voltage; .vertline.At.vertline. designates an antiferroelectric threshold voltage; and .vertline.As.vertline. an antiferroelectric saturation voltage.
Generally, it is often the case that the curves (namely, the hysteresis curves) of FIG. 16 representing the optical transmittance characteristics of a liquid crystal relative to the voltage applied thereto are obtained by applying thereto a triangular-wave-like voltage which is generated in such a manner that the absolute value of the ratio of a change in this voltage relative to time, namely, the value of .vertline.dV/dt.vertline. is constant. However, in this case, if the value of .vertline.dV/dt.vertline. is changed, the shapes of the hysteresis curves also change. Moreover, the values of the aforementioned quantities As, Ft, Fs and At also vary. It is, accordingly, necessary to specify these values to specify the aforesaid value of .vertline.dV/dt.vertline.. However, in the case of the device of the present invention, data concerning the graph of FIG. 16 is obtained by the following method so as to obtain values of quantities corresponding to actual driving conditions. In this case, it is assumed that the temperature thereof is the working temperature.
Moreover, it is further assumed that the duration of one frame (to be described later) is Pt and that the length of a time period in which a selection voltage (to be described later) is applied to an liquid crystal, is Wt.
(1) A pulse voltage, whose duration is Wt and voltage level is Vz, is applied to the liquid crystal that is in a stable antiferroelectric state (namely, in the neutral state). Further, the relationship between the optical transmittance and the pulse voltage Vz at the time of completion of the application of this pulse voltage is plotted. Moreover, this operation is repeated by changing the value of the voltage Vz. Thereby, the curve drawn from the point O to the transmittance corresponding to the voltage Fs through the transmittance corresponding to the voltage Ft of FIG. 16, as well as the curve drawn from the point O to the transmittance corresponding to the voltage (-Fs) through the transmittance corresponding to the voltage (-Ft), is obtained. PA1 (2) Next, the liquid crystal is first put into the saturated ferroelectric state by applying thereto a voltage which is not lower than the aforementioned voltage .vertline.Fs.vertline.. Then, at a moment 0, the applied voltage is reduced to .vertline.Vz.vertline.. Thence, after the elapse of the time period of the length (Pt-Wt), the relation between the optical transmittance and the applied voltage Vz is plotted. Moreover, this operation is repeated by changing the value of the voltage .vertline.Vz.vertline.. Thereby, the curve drawn from the transmittance corresponding to the voltage Fs to the point O through the transmittances respectively corresponding to the voltages At and As of FIG. 16, as well as the curve drawn from the transmittance corresponding to the voltage (-Fs) to the point O through the transmittances respectively corresponding to the voltages (-At) and (-As), is obtained.
When some liquid crystal panels are used, the curve (namely, the curve drawn from the transmittance corresponding to the voltage Fs or (-Fs) to the point O in FIG. 16) obtained in the aforementioned case (2) sometimes intersects the ordinate axis. The main cause of this is the responsivity of the liquid crystal. Namely, in the case that the liquid crystal is maintained in the ferroelectric state by applying thereto a voltage, which is not lower than the aforementioned voltage .vertline.Fs.vertline., and that at the moment 0, the applied voltage Vz is changed into 0, the liquid crystal finally becomes stable in the antiferroelectric state after the elapse of a certain time period (hereunder referred to as a relaxation time tn). However, if this relaxation time tn is longer than the time period (Pt-Wt), the curve obtained in the aforementioned case (2) intersects with the axis of ordinate.
When actually driven, it is difficult to bring such a liquid crystal panel into a completely antiferroelectric state. It is, thus, considered that in the case of such a liquid crystal panel, a dark display cannot be effected and that the contrast is extremely degraded.
Generally, a liquid crystal panel is driven by performing the following process. Namely, first, N row electrodes and M column electrodes are formed in such a manner as to be arranged as a matrix of N rows and M columns. Further, a scanning signal is applied to each of the row electrodes through a row-electrode drive circuit, while a display signal depending on display data representing each pixel (incidentally, a part of data represented by the display signal is sometimes not dependent on the display data) is applied to each of the column electrodes through a column-electrode drive circuit. Moreover, a voltage (hereunder referred to simply as a synthesis voltage), which corresponds to the difference between the scanning signal and the display signal, is applied to a liquid crystal layer. Thus, the liquid crystal panel is driven. The time period required to scan all of the row electrodes (namely, 1 vertical scanning interval) is usually referred to as 1 frame (or 1 field). In the case of driving the liquid crystal panel, the polarity of a driving voltage is reversed or inverted each frame (or every frames) in order to prevent the liquid crystal from being adversely affected (namely, prevent the degradation of the liquid crystal due to a non-uniform distribution of ions).
Paying attention to the scanning signal to be applied to a single row electrode, 1 vertical scanning interval is composed of N horizontal scanning intervals (in some case, an additional interval is added thereto). Among a horizontal scanning interval, a part of horizontal scanning interval in which a scanning voltage (hereunder referred to as the selection voltage) to be used for determining the display condition of a pixel on this row is applied, is referred to as a selection period tw. The other part of horizontal scanning interval are referred to as non-selection periods.
FIG. 17 illustrates the waveforms of signals flowing through the row electrodes, the column electrodes and the pixel synthesis electrodes of a liquid crystal panel in which the N row electrodes and the M column electrodes are formed in such a manner as to be arranged as a matrix of N rows and M columns. The display conditions or states of pixels are assumed to be as follows. Namely, in the case of a first column (Y1), pixels respectively corresponding to intersections with all rows are displayed in white. Further, in the case of a second column (Y2), a pixel corresponding to an intersection with a first row is displayed in black. Pixels respectively corresponding to intersections with the other rows are displayed in white. Moreover, in the case of pixels in a third column (Y3), these pixels respectively corresponding to intersections with all rows are displayed alternately in black and in white. Furthermore, in the case of an Mth column YM, pixels respectively corresponding to intersections with all rows are in a black display state, namely, displayed in black.
Scanning signals are respectively applied to the N row electrodes in sequence from the top row to the bottom row so that the waveforms of the scanning signals respectively corresponding to the adjacent row electrodes are shifted by a phase corresponding to a time that is (1/N) of the frame interval. Display signals are respectively applied to the M column electrodes so that the waveform of the display signal applied to each of the column electrodes is synchronized with that of the scanning signal applied thereto and is generated according to the display conditions of the pixels corresponding thereto, namely, according to whether the pixels are displayed in white or in black.
Turning to a synthesis voltage corresponding to each pixel, the voltage applied to a pixel P11, which is displayed in white in a first row, in the selection period tw is large, whereas the voltage applied to a pixel P12, which is displayed in black in the first row, in the period tw is small. The other part of the synthesis voltage has the same waveform. The synthesis voltage applied to a pixel P21, which is displayed in white in a second row, has a waveform which is almost the same as that obtained by shifting the waveform of the synthesis voltage applied to the pixel P11 by the phase corresponding to a time period that is (1/N) of the frame interval. Here, note that the first frame and second frame in the first row and second row are shifted each other by (1/N).
Usually, in the case of the antiferroelectric liquid crystal panel, it is determined on the basis of the aforementioned display signal at the time of applying the selection voltage whether the state of the liquid crystal, which has been in the antiferroelectric state, is maintained or is changed into the ferroelectric state. Thus, there is the necessity of a time period (hereunder referred to as a relaxation period ts) required for setting the liquid crystals in the antiferroelectric state before the application of the selection voltage. During a time period which is other than the selection period tw and the relaxation period ts, the determined state of the liquid crystal should be held. Hereunder, this time period will be referred to as a holding or keeping period tk.
Further, there are two kinds of known driving systems, namely, a system in which the aforementioned relaxation period ts is provided in the aforesaid selection period tw (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 4-362990/1992), and another system in which the aforementioned relaxation period ts is provided in a time period (namely, the non-selection period) that is other than the aforesaid selection period tw (see, for instance, Japanese Unexamined Patent Publication (Kokai) No. 6-214215/1994).
FIG. 18 illustrates the waveforms of a scanning signal Pa applied to a given pixel of interest, display signals Pb and Pb', synthetic signals Pc and Pc' and optical transmittances L100 and L0 according to the driving method described in FIGS. 1 and 2 of the aforementioned Japanese Unexamined Patent Publication (Kokai) No. 4-362990/1992.
In FIG. 18, reference characters F1 and F2 designate the first frame and the second frame, respectively. This figure illustrates the case where the polarity of the aforementioned driving voltage is reversed every frame. As is apparent from this figure, the first frame F1 is different from the second frame F2 only in that the polarity of the driving voltage is inverted. As is obvious from the aforementioned FIG. 16, the operation of the liquid crystal display device is symmetrical with respect to the polarity of the driving voltage. Therefore, the following description will be given regarding only the first frame, unless descriptions concerning the second frame are necessary. Further, the description concerning the second frame, which is different from the first frame only in the polarity of the applied voltage, is omitted herein.
Further, in the following description and drawings of the waveform of driving signals, the electric potential indicated as "0" does not mean absolute electric potential but means mere reference electric potential. Therefore, in the case that the reference electric potential varies for some reason, scanning signals and display signals vary relatively. Moreover, in the case that the word "voltage" is used in connection with the scanning signals and the display signals in the following description, the word "voltage" designates the difference between the electric potential indicated by such a signal and the reference electric potential.
As shown in FIG. 18, 1 frame is divided into three time periods, namely, the selection period tw, the holding period tk and the relaxation period ts. The selection period tw is further divided into time periods tw1 and tw2, which have equal lengths. The voltage level of a scanning signal Pa in the first frame F1 is set as follows. Needless to say, in the second frame F2, the polarity of the voltage is inverted. Here, note that .+-.V designates the selection voltage and that the length of the time period tw2 corresponds to the duration Wt of the pulse voltage.
______________________________________ Time Period tw1 tw2 tk ts ______________________________________ Scanning Signal Voltage 0 +V1 +V3 0 ______________________________________
Further, the display signal is set as follows. Here, note that the symbol "*" indicates that the voltage depends on the display data representing other pixels in a same column as this pixel.
______________________________________ Time Period tw1 tw2 tk ts ______________________________________ On-State Display Signal Voltage +V2 -V2 * * Off-State Display Signal Voltage -V2 +V2 * * ______________________________________
In the case of the hysteresis curves of FIG. 16, generally, the curve drawn from the transmittance corresponding to the voltage As to the transmittance corresponding to the voltage Ft or from the transmittance corresponding to the voltage At to the transmittance corresponding to the voltage Fs is not flat. Thus, when the voltage applied to the liquid crystal in the holding period tk is shifted depending on the display signal, a change in the brightness in this holding period is caused. To prevent an occurrence of this phenomenon, usually, the polarity of the display signal is inverted in such a manner that the average value thereof in a horizontal scanning interval is 0. Namely, the time period tw1 is different from the time period tw2 in that the polarity of the display signal is inverted. Hereunder, a time period, in which a display signal should be applied thereto according to the display data (incidentally, the waveform of the signal varies with the display data) in all of the time periods (namely, the selection period, the holding period and the relaxation period) will be referred to as a display signal active period. For example, in the case of FIG. 18, one period or cycle of the display signal Pb consists of a time period, in which this signal has a signal level of +V2, and another time period in which this signal has a signal level of (-V2). Thus, the signal voltage of +V2 or (-V2) is applied thereto at all times, so that all of the time periods are the display signal active period. However, in the case of FIG. 19 as will be described in detail later, the period or cycle of the display signal Pb consists of a time period, in which the signal voltage is 0, and another time period, in which this signal has a signal level of +V2, and still another time period in which this signal has a signal level of (-V2). Thus, the time period in which the display signal is applied thereto is those in which the signal voltages of +V2 and (-V2) other than 0 are applied thereto. Consequently, these time periods are the display signal active period in this case.
In FIG. 18, reference characters Pb, Pc and L100 respectively denote the waveform of a display signal, that of a synthetic signal and optical transmittance in the case that all of the pixels provided on a column electrode, to which a pixel of interest belongs, are in an on-state (namely, in a bright or light state). In this case, if the (synthetic) voltage to be applied to the liquid crystal in the time period tw2 meets the following condition: .vertline.V1+V2.vertline.&gt;.vertline.Fs.vertline. (see FIG. 16), the transition of the state of the liquid crystal into the ferroelectric state is started. As a result, the optical transmittance of the liquid crystal increases. In the holding period tk, if the following condition is satisfied: .vertline.V3-V2&gt;.vertline.&gt;.vertline.At.vertline., the bright state is held. In the relaxation period ts, if the following condition is satisfied: .vertline.V2 .vertline.&lt;.vertline.As.vertline., the transmittance decreases with the elapse of time. Thus, the relaxation of the liquid crystal, namely, the change of the state thereof from the ferroelectric state to the stable antiferroelectric state is attained.
Further, in FIG. 18, reference characters Pb', Pc'and L0 respectively designate the waveform of a display signal, that of a synthetic signal and optical transmittance in the case that all of the pixels provided on a column electrode, to which a pixel of interest belongs, are in an off-state (namely, in a dark state). In this case, if the synthetic voltage to be applied to the liquid crystal in the time period tw2 meets the following condition: .vertline.V1-V2.vertline.&lt;.vertline.Ft.vertline., the voltage applied in the holding period tk meets the following condition: .vertline.V3+V2.vertline.&lt;.vertline.Ft.vertline., and the voltage applied in the relaxation period ts meets the following condition: .vertline.V2.vertline.&lt;.vertline.Ft.vertline., the dark state is held.
FIG. 19 is a waveform diagram illustrating the waveform of a driving signal used in the driving method that is described in Japanese Unexamined Patent Publication (Kokai) No. 6-214215/1994. In the case of this driving method, 1 frame is divided into the selection period tw and the holding period tk. The selection period tw is further divided into three time periods, namely, two time periods tw1 and tw2, which have equal lengths, and a time period two which precedes the two periods tw1 and tw2. In the case of this driving method, the aforementioned relaxation period ts is the aforesaid time period tw0. Further, the voltage level of a scanning signal and the display signals in the first frame F1 are set as follows.
______________________________________ Time Period tw0 tw1 tw2 tk ______________________________________ Scanning Signal Voltage 0 0 +V1 +V3 On-State Display Signal Voltage 0 +V2 -V2 * Off-State Display Signal Voltage 0 -V2 +V2 * ______________________________________
In the case of the driving method described in Japanese Unexamined Patent Publication (Kokai) No. 6-214215/1994, the zero-volt voltage applying time period (tw0) provided in the leading part of the selection period tw is used as the relaxation period ts. In this case, a time period having the length (tw-tw0), namely, a part of the period tw, which is other than the time period tw0 during when the display signal voltage is 0, is the display signal active period. In the case of this driving method, when using a liquid crystal panel whose relaxation time tn is long, the state thereof cannot be changed into the antiferroelectric state unless the length of the period tw0 is sufficiently increased. Hence, the panel cannot help but increase the length of the selection period tw (namely, 1 horizontal scanning interval). Thus, in the case of a highly fine display device (whose horizontal scanning interval is short if the frame frequency is constant), inconveniences are caused.
Regarding this respect, in the case of the driving method illustrated in FIGS. 1 and 2 of Japanese Unexamined Patent Publication (Kokai) No. 4-362990/1992, a plurality of horizontal scanning intervals in the nonselection period are utilized as the relaxation period ts. Thus, even if the relaxation period tn of the liquid crystal panel is large, there is no necessity of increasing the length of the selection period tw. However, the optical transmittance of a liquid crystal in the relaxation period ts does not reflect display data correctly. Further, the optical transmittance thereof is unstable toward a change in temperature. Therefore, if the ratio of the relaxation period ts to one frame is large, a favorable display cannot be obtained. For instance, in the case that a time period corresponding to 1 pixel (namely, a total of the lengths of the selection period, the holding time and the relaxation time) in 1 frame is 20 msec, and if the relaxation time should be at least 18 msec, 90% of the length of 1 frame is occupied by an unstable display time. Consequently, it becomes difficult to obtain a favorable display.